The present invention relates to etching process during the production of semiconductor devices. More specifically, the invention relates to etching process for semiconductor devices with a layer of a high-k dielectric constant material.
Conventionally, an ONO (oxide nitride oxide) layer has been used in flash memory gate stack for memory storage. However, the dielectric constant of ONO is not enough to meet the ever increasing demand in operation voltage. Accordingly, high dielectric constant material (also referred as to high-k dielectric material) has been introduced to replace ONO.
The dielectric constant of SiO2 is about 3.9. If high k material like Al2O3 (aluminum oxide) is used to replace SiO2, the dielectric constant will increase to around 9.0. Other than Al2O3, HfO2, and Ta2O3 are also considered as candidates for high-k materials in flash memory gate stack to replace ONO. Among them, Al2O3, HfO2 and Al2O3/HfO2/Al2O3 sandwich structure have been used.
However, etching of high-k dielectric material has been found to be more difficult compared to etching ONO, because of lower volatility of its etch byproduct. Because of this, the etch rate and the selectivity to polysilicon film of high-k dielectric material have been found to be much lower compared to ONO film. Efforts have been made to increase the etch rate and selectivity of high k material to polysilicon. In order to achieve good etch selectivity to polysilicon, conventional BCl3 based etch chemistry requires low bias voltage. However, such low-biased BCl3 process has severe micro loading problems.